Two gliders collide on a frictionless air track that is aligned along the x axis. Glider A has an initial velocity of +4.0 m/s and a mass of mA = 0.20 kg. Glider B is initially at rest and has a mass mB. After they collide, A has a velocity of -1.0 m/s and B has a velocity of +2.0 m/s. Determine mB. Assume that It is an elastic collision.